Satellite broadcasting systems for transmitting programming content have become increasingly popular in many parts of the world. Direct Broadcasting Satellite (DBS) systems transmit television programming content, for example, to a geo-stationary satellite, which broadcasts the content back to the customers. In such a wireless broadcast environment, the transmitted programming can be received by anyone with an appropriate receiver, such as an antenna or a satellite dish.
In addition, a number of satellite broadcasting systems have been proposed or suggested for broadcasting audio programming content from geo-stationary satellites to customers in a large coverage area, such as the continental United States. Proposed systems for providing digital audio broadcasting (DAB), for example, are expected to provide near CD-quality audio, data services and more robust coverage, than existing analog FM transmissions. Satellite broadcasting systems for television and radio content provide potentially national coverage areas, and thus improve over conventional terrestrial television stations and AM/FM radio stations that provide only regional coverage.
Satellite broadcasting systems transmit digital music and other information from an uplink station to one or more mobile receivers. Satellite broadcasting systems typically include a plurality of satellites and terrestrial repeaters operating in a broadcast mode. The satellites are typically geo-stationary, and are located over a desired geographical coverage area. The terrestrial repeaters typically operate in dense urban areas, where the direct line of sight (LOS) between the satellites and the mobile receiver can be blocked due to the angle of elevation and shadowing by tall buildings.
Orthogonal frequency division multiplexing (OFDM) techniques have also been proposed for use in such satellite broadcasting systems and other wireless networks. In an OFDM communication system, the digital signal is modulated to a plurality of small sub-carrier frequencies that are then transmitted in parallel. It has been found that OFDM communication systems do not require complex equalizers, even at high data rates and under multipath propagation conditions. Among other benefits, OFDM communication systems provide a guard interval that absorbs the multipath distortion into the guard interval duration. As long as the arrival times of the multipath signals differ from one another by less than the guard interval, an equalizer is not necessary.
OFDM communication systems are especially sensitive to frequency offsets in the carrier signal. Typically. OFDM systems use additional pilot and/or synchronization signals to perform the initial acquisition of the carrier signal. One popular technique utilizes training sequences over two consecutive OFDM symbols. While this technique acquires the carrier signal in a satisfactory manner, it suffers from a number of limitations, which if overcome, could greatly expand the efficiency of OFDM communication systems. More specifically, prior techniques relying on additional pilot and/or synchronization signals reduce the effective OFDM capacity for carrying data, which is unsatisfactory for capacity-limited systems.
Another technique for acquiring the carrier signal, referred to as the Guard Interval Based (GIB) algorithm, can only distinguish to within one half of the inter-carrier spacing. Thus, the GIB algorithm is insufficient for estimating frequency offsets that are greater than one half of the inter-carrier spacing, which are typical at system startup. For a more detailed discussion of the GIB algorithm, see, for example, Jan-Jaap van de Beek et al., ML Estimation of Time and Frequency Offset in OFDM Systems, IEEE Transactions on Signal Processing, Vol. 45, No 7, 1800-05 (July 1997) or Jan-Jaap an de Beek et al., “A Time and Frequency Synchronization Scheme for Multiuser OFDM,” IEEE J. on Selected Areas in Communications, Vol. 17, No. 11, 1900-14, (November 1999), each incorporated by reference herein.
U.S. patent application Ser. No. 09/382,847, filed Aug. 25, 1999, entitled “Orthogonal Frequency Division Multiplexed (OFDM) Carrier Acquisition Method,” assigned to the assignee of the present invention and incorporated by reference herein, discloses an improved technique, referred to herein as the modulo-subcarrier (MODSC) algorithm, for acquiring the carrier signal. The MODSC algorithm inserts a spectral null in the transmitted OFDM signal at a predefined location, locates the spectral null at the receiver and uses the shifting of the spectral null to estimate the local oscillator carrier offset. The location of the detected null estimates the carrier offset in units of the number of intercarrier spacings. Among other benefits, the MODSC algorithm does not require the use of additional pilot signals and thus optimizes the bandwidth utilization.
While the MODSC algorithm accurately determines the carrier offset without reducing the effective bandwidth utilization, a further need exists for a method and apparatus that acquires the carrier signal and also declares when the carrier signal has been acquired or when a false-lock or out-of-lock condition has occurred. Another need exists for a method and apparatus that acquires the carrier signal and that provides for reliable transitions between acquisition and tracking modes.